Surface hardening of ferrous metal by self-quenching



July 29, 1941 G. H. BROWN 2,424,794

SURFACE HARDENING .QF FERROUS METL BY sELF-QUENCHING Fi'led Dec. 5, 1941 j, fyi

Patented July 29, 1947 SURFACE HARDENING OF FERROUS METAL BY SELF-QUENCHING George H. Brown, Haddoneld, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application December 5, 1941, Serial No. 421,746

3 Claims. l

This invention relates to the heat treatment of metals, and more particularly to a method of hardening ferrous metals.

The use of ferrous metal parts in many inn dustrial applications and other fields of use is well known. Where such parts involve surfaces which rub against each other, as in the case of a piston in the cylinder bore of an internal combustion engine, for example, these surfaces are subject to considerable wear, and in order to prolong the life of the parts involved, it has been necessary to harden the surfaces. Now, it is known that when ferrous metals are heated to suitable temperatures, the structure thereof is changed; and if the metal is quenched or cooled relatively quickly, it becomes hard instead of returning to its relatively softer state as would be the case if it were cooled slowly.

In accordance with prior practices, it has been customary to heat the metal to be hardened either by means of an open iiame orby use of high frequency electrical energy at low powerI and to quench it either by exposure to air, by spraying cold water thereon, or by immersion in water, oil or other suitable fluid. 'Ihese practices have several disadvantages. In the first place, heating the metal by means of a flame or by use of high frequency electrical energy at low power involves the application of heat relatively slowly and generally causes the metal to become hot over a substantial depth beyond its surface, thus producing a hardened layer of substantial thickness. Since the hardened metal is much more brittle than the softer metal and, hence, much more frangible, this is undesirable. Furthermore, the distribution of heat through the work treated in this manner usually causes the work to Warp, and this, obviously, is also undesirable. In the second place, the use of air, water or oil as a quenching medium does not insure sufficiently rapid quenching necessary in many instances to harden the parts. This is largely for the reasons that (l) these and other similar fluids which have been employed for quenching are not as good conductors of heat as the metal itself, and (2) these fluids, at best, are not in as intimate contact with the metal as the metal particles themselves are in contact with each other. Moreover, quenching by prior art methods has the further disadvantage of producing scale. It is apparent, therefore, that the methods of hardening metals practiced heretofore fall considerably short of optimum results.

The primary object of my present invention is to provide an improved method of hardening ferrous metals which will not be subject to the aforementioned disadvantages.

More particularly, it is an object of my present invention to provide an improved method of treating ferrous metals which will produce hardened surfaces therein with great rapidity.

Another object of my present invention is to provide an improved method of treating ferrous metals as above stated wherein the depth of the hardened surface layer will be a minimum and its hardness a maximum.

Still another object of my present invention is to provide an improved method of treating ferrous metals as aforesaid whereby the metal will be heated to a uniform depth.

A further object of my present invention is to provide an improved method of treating ferrous metals as above set forth whereby no scale will be produced.

It is also an object of my present invention to provide an improved method ofv treating ferrous metals as aforesaid which may be carried out with great facility and with very high efficiency.

In accordance with this invention, I employ electrical energy at radio frequency as a source of heat and apply the heat to the surface of the work to be hardened by electromagnetic induc-v tion through a suitable applicator, such as an inductor ring or other suitabile coupling coil. By using high frequency electrical energy at suiciently high power, extremely rapid heating is obtained, and by scanning the Work with the applicator at a suitable rate (Which is made possible by this rapid heating), the extreme heat developed will have time to penetrate into the work for only a very small distance from the surface and uniformly throughout the entire surface during the time that the heat'is applied. In this way, adequate heating and small penetration of the heat are effected. Subsequently, instead ofA employing air, Water or oil for quenching the heated material, I permit the heat to be dissipated through the metal itself which backs up this heated'layer adjacent the surface at a rate which is of the order of the rate at which the heat is applied. Thus, the heated work undergoes self-quenching, so to speak, and the developed heat is dissipated much more rapidly than it is according to customary practices heretofore employed. Where the work is of substantial thickness, the portion thereof in back of the thin, heated layer will ordinarily be adequate to conduct away the heat with sufficient rapidity. However, where the work is very thin, it may -be necessary to back the work up with a metal of relatively low melting point, such as Woods metal or solder, and as the heat passes therethrough, the backing metal will melt away while dissipating the heat (or, if necessary, it vmay be heated later to melt it) or it may be removed in any other suitable manner. In any event, the work will have acquired, during a very short time interval, a very hard, thin surface layer which is entirely suitable to commercial needs, as compared with relatively longer time intervals required by prior art methods and the less desirable, thicker, hardened layers which they produce.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, as Well as additional objects and advantages thereof, will best be understood from the following description, when read in connection with the accompanying drawing, in which Figure l is a view, partly in section, of a system employing my present invention in connection with the block of an internal combustion engine wherein the cylinder bores are formed in a solid block,

Figure 2 is a fragmentary sectional view showing the application of my present invention to a cylinder block having thin walls and of hollow construction, as used, for example, in airplane engines,

Figure 3 is a plan view showing the application of my present invention to a spur gear for hardening the teeth thereof,

Figure 4 is view .partly in elevation and partly in section taken along the plane of line IV-IV of Figure 3,

Figure 5 is a plan view showing the application of my present invention to a Irod or shaft, and

Figures 6 and 7 are curves showing the rapidity with which materials of various thicknesses may be hardened in accordance with my present invention and at different power inputs.

Referring more particularly to the drawing, wherein similar reference characters designate corresponding parts throughout, I have shown, in Figure 1, a high frequency oscillation generator l which supplies energy through a current transformer 3 to an inductor ring 5 connected to the secondary winding of the transformer 3 and constituting a coupling coil around which a concentrated, electromagnetic field is set up and which is adapted to heat to a predtermned ternperature a very thin layer under the surface of the metal to be treated. In the arrangement shown in Figure 1, the work or metal to be treated is constituted by a solid engine block 1 of an internal combustion engine having a plurality of cylinder bores 9 the walls of which are to be hardened.

The transformer 3 and the inductor ring or applicator 5 may be of the type disclosed in my copending application Serial No. 390,628, filed April 26, 1941, which has matured into Patent Number 2,348,325, the applicator 5 and the engine lblock 1 being arranged so that the `applicator 5 nts within one or the other of the bores 9 in closely spaced relation thereto. Any suitable mechanism may be employed to provide relative movement between the inductor ring or applicator 5 and the engine block 1. For this purpose, the block 'I may be placed upon a platforml guided for vertical movement by guide blocks i3 and associated shafts l5 depending from the platform Il and slidably received in suitable openings in the guide blocks I3. Vertical movement is imparted to the platform H and the block 1 by a, feed screw I1 received in the internally threaded hollow hub IS formed on the platform Il, the feed screw I1 being connected to a gear 2| which is driven from a suitable motor 23 through a gear 25.

Engine blocks of the type under consideration are usually formed of ferrous metals of high carbon content and are usually very tough, although relatively soft. It is for this reason that the cylinder Walls must be hardened. In accord,-

ance with my present invention, the hardening of the cylinder walls can be eected by applying heat thereto by electromagnetic induction from the coupling coil or applicator 5 at such a rate that a layer of the metal under the surface of the bores not substantially in excess of 0.020 inch in thickness will be substantially instantaneously heated to a predetermined temperature depending upon the composition of the metal. In one case, for example, the metal operated upon required a temperature of 950 degrees centigrade for proper hardening. To accomplish this, the oscillator I was operated at a frequency of kilocycles and delivered power to the work at the rate of about 5 kilowatts per square .centimeter through an inductor ring 5 about 1/8 inch thick. As the block was moved relative to the applicator 5 at a speed of about 11/2 inches per second, a cherry red ring which traveled along with the inductor ring 5 on the cylinder wall could be observed. As a `result of this treatment, the cylinder walls were heated almost instantaneously to a uniform thickness of approximately 0.020 inch. I have found that a heated layer or skin of from approximately 0.005 inch to approximately 0.020 inch forms a sufficiently thick, hardened layer to take care of any reasonable wear during the life of the engine block.

After the work has been heated rapidly' as above described, it is essential that it be quenched rapidly, as well, in order to harden the heated layer or skin as above described. For this purpose, I permit the heat developed in the aforementioned heated layer to be dissipated rapidly through the balance ofl the engine block. Since the metal Which backs up the heated layer is in very intimate contact therewith and is an excellent conductor of heat, it is clear that the backing .portion of the metal will readily dissipate the heat, this dissipation taking place down through the reoalescence point of the metal at a rate which is of the order of that at which the heat is applied. The resulting structure is one in which the heated surface layers are extremely hard,

As described above in connection with Fig, 1, it is assumed that the metal being treated is of infinite thickness, and by this I mean a thickness in excess of approximately one-half inch. In some cases, it is desirable to harden the surface of metal which is much thinner. For eX- ample, in airplane engines, the engine blocks are hollow and are made of metal which may be 'approximately one-eighth inch thick. Applying my improved process as above described to such a thin layer may result in hardening most of the thickness thereof. To avoid this, I preferably employ the method illustrated in Fig. 2 wherein the hollow block 1 of relatively high melting point ferrous metal is provided with one or more openings 21 through which a low melting point metal 29, such as Woods metal or solder, may be poured to back up the thin walls of the cylinders 9, after which the openings 21 may be closed by suitable plugs 3|. The heat may be applied to the unbacked, exposed Walls of the bores 9 as above described in connection with Fig. 1 and to a hardening temperature in'- termediate the aforementioned two melting points so that a layer of a thickness not substantially in excess of 0.020 inch will be heated substantially instantaneously as the block 1 is moved relative to the inductor ring 5. The backing metal 29, being in intimate contact with the back surfaces of the cylinder walls, will rapidly conduct away, down through the recalescence point,

the heat applied to the thin, surface layers at a rate substantially equal to the rate at which the heat was applied and thus effect the required rapid quenching.

Thus far, my improved process has been described with reference to work which surrounds the inductor ring 5. However, the process may be applied equally well to hardening of the outer surfaces of Various articles by having the inductor ring surround such articles. Illustrations of such applications are shown in Figs. 3 and 4 wherein the inductor ring 5 surrounds a spur gear 33 for hardening the teeth thereof, and in Fig. 5 wherein the inductor ring 5 surrounds a shaft or rod 35 along which it maxr be moved at a suitable rate for hardening the outer surface of the shaft. In any case, the heat is applied with sufficient rapidity to cause only a thin layer to be substantially instantaneously heated to the desired temperature as above described, the heat applied to said layer being thereafter rapidly quenched by dissipation through the metal which backs up the heated layer.

In Figs. 6 and 7 are shown two sets of curves illustrating the advantages of my present invention, the curves of Fig. 6 relating to a metal of infinite thickness, and the curves of Fig. 7 relating to a thin metal of about one-half centimeter, or one-eighth inch, in thickness. In both sets of curves, temperature is plotted against time, the former being represented by the ordinates, and the latter by the abscissae. The curve A of Fig. 6 shows that if high frequency energy is applied at 1000 watts per square centimeter to the surface of a ferrous metal which must be heated to 950 degrees centigrade for proper hardening, it will take about one and one-half seconds to reach that temperature. Thereafter, quenching takes place at a relatively slow rate, even when the backing metal is relied upon, and atemperature of about 450 degrees in the heated layer, which is the point of recalescence of the particular metal worked on, is not reached until about two and one-half seconds after starting the application of heat. On the other hand, if that is applied at 5000 watts per square centimeter, a temperature of 950 degrees is reached in about .075 second, as shown by the curve B in Figl 6, and the heated layer is quenched down to almost 200 degrees in a total of about 0.4 second. The advantage of my improved method can be still more appreciated when it is considered that, in connection with the curve A, quenching would be still slower if effected by means of amr of the fluids commonly used according to prior art practices.

In Fig. 7, the curve C represents a power input at 1000 watts per square centimeter and shows that the temperature of 950 degrees is reached in slightly over 0.6 second, since the metal being worked on is very thin, and the metal is quenched to about 800 degrees in a total of approximately .75 second, after which further quenching is exceedingly slow. As practiced in accordance with my improved method, however, and as shown by the curve D, if power is applied to the thin metal at 5000 watts per square centimeter, the heated layer will reach a temperature of 950 degrees in about .075 second and will quench to somewhat under 400 degrees in a total of about 0.15 second from the time that application of heat is started. In the case of both the curve B and the curve D, it will be noted that the rate which heat is applied up to the hardening temperature and the rate at which it is later removed down through the recalescence point is substantially the same.

Although I have shown and described my improved process in considerable detail, it will undoubtedly be apparent -to those skilled in the art that various modifications and changes may be made therein within the spirit of the invention. For example, while I have specified an operating frequency of 100 kilocycles for the oscillator il as one example and have suggested a power input at 5000 watts per square centimeter, it will readily be appreciated that these may be varied over a wide range to suit particular requirements. Moreover, instead of employing a thin inductor ring 5 as the applicator and moving it or the work relative to one another, the applicator may be made equal in length to that of the work and both held stationary during application of heat. Other similar changes will, no doubt, readily suggest themselves to those skilled in the art.

I claim as my invention:

1. The method of treating a relatively thin, ferrous metal of high melting point which comprises backing said metal up with a second metal having good heat conductivity and a much lower melting point than said first named metal, then heating the exposed surface of said first named metal to a predetermined, critical temperature intermediate said melting points with such rapidity that the heat applied to said first named metal over a given area will not have suilicient time to pass on through said first named metal to said second named metal during application of the heat to said area, and subsequently causing said first named metal to be quenched by dissipating the heat applied thereto throughout said second named metal.

`2. The invention set forth in claim 1 characterized in that said first named metal is heated by applying thereto high frequency electrical energy by electromagnetic induction.

3. The method of hardening relatively thin, high melting point ferrous metal which comprises backing said ferrous metal up with a second metal of relatively low melting point, then applying to the unbacked surface of said ferrous metal high frequency electrical energy by electromagnetic induction at a rate such as to substantially instantaneously heat only a layer thereof under said unbacked surface not substantially in excess of 0.020`inch to a hardening temperature intermediate said melting points, and finally causing said heated layer to 'be quenched rapidly by dissipating the heat in said layer throughout said secon-d named metal.

GEORGE H. BROWN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS- Name Date 2,273,809 KinZel Feb. 17, 1942 2,178,201 Dake Oct. 31, 1939 2,202,758 Denneen et al May 28, 1940 2,208,607 Somes July 23, 1940 2,223,902 Somes Dec. 3, 1940 2,238,082 Somes Apr. 151, 1941 2,256,873 Somes Sept. 23, 1941 FQREIGN PATENTS Number Country Date 467,308 Great Britain June 15, 1937 

